Studies will be carried out to gain a clearer understanding of Toxoplasma and toxoplasmosis. Since intracellular organisms are causing increasing morbidity and morality in immunosuppressed patients, the immunological aspects of Toxoplasma and toxoplasmosis are being studied to define the antigenic nature of the organism as well as the nature of the immune response and the relative importance of cells versus conventional circulating antibody in immunity against intracellular organisms per se. To gain a better understanding of congenital toxoplasmosis the mechanism of transmission from acutely and chronically infected mothers (both human and animal) as well as the role of maternal antibody in inhibiting an immune response in the infected fetus is being studied. The important area of infection in the immunologically compromised host is being studied using Toxoplasma as a tool to study resistance to intracellular infection. The largest single group of patients which we are studying at present are Hodgkin's disease and cardiac transplant recipients. The variability in resistance to infection among various population groups and the differences noted by sex led us to develop a model with Toxoplasma and inbred strains of mice. A major effort is being made to define the genetics of host resistance and susceptibility to Toxoplasma and the underlying mechanisms. BIBLIOGRAPHIC REFERENCES: Hyde, B., Barnett, E.V., and Remington, J.S.: Method for differentiation of non-specific from specific Toxoplasma IgM fluorescent antibodies in patients with rheumatoid factor. Proc. Soc. Exp. Biol. Med. 148:1184-1189, 1975. Remington, J. and Desmonts, G., in Remington, J. and Klein, J. (Eds): Infections of the fetus and newborn infant, Philadelphia; W.B. Saunders, 1976.